With the fast development of manufacturing technologies, more and more structures of micro part with dimensions in the range of 0.1-1 mm and aspect ratios higher than 10:1 are now used for an increasing number of applications, including ink-jet printer nozzles, microgroove array in aerospace propulsion engines, cooling vents in turbine blade and diesel fuel injection holes, which present challenges to the measurement precision and measurable depth of existing probing systems. Therefore, it is of significance to develop a precise probing system for the equipment for coordinate measurement of a micro part, especially for one with a miniaturized size and high measurable aspect ratio.
Much work has been done on this particular aspect in recent years. For example, Gaoliang Dai, Sebastian Biitefisch, Frank Pohlenz and Hans-Ulrich Danzebrink et al. have invented a small silicon probe based on MEMS fabrication process. This probe consists of a silicon chip membrane and integrated piezoresistive elements. The piezoresistive elements are etched onto the silicon membrane to detect three-dimensional deformation, and the stylus is attached to the center of the silicon membrane. The diameter of the probe tip is less than 300 μm and probing force achieved by the membrane system is around 100 mN. However, the fabrication process is complex and produce cost is high.
Owing to the low produce cost, immune to electromagnetic interference and interruption, and lightweight and size small, more and more fiber probes are developed to achieve dimensional measurement. H. Schwenke, F. Wäldele, C. Weiskirch, H. Kunzmann have invented a fiber probe with a fiber sphere tip to backscatter the light. The stylus of this probe is 15 μm in diameter, and the spherical tip is 25 μm in diameter. The laser beam enters through the fiber and is incident on the fiber sphere tip. The back scattered light is imaged using a CCD camera, and contact displacement in xy-direction is thus transformed into the change of the center of light spot in CCD camera. This probe can be extended to a three-dimensional system by attaching a fiber sphere in the stylus and the image of this sphere is reflected on a second CCD camera using a mirror. But due to shadowing effect, CCD camera cannot obtain enough light energy to create an image, and the inspecting depth achievable with this probing method is thus limited.
Jiubin Tan and Jiwen Cui have invented a spherical coupling optical fiber probe. The spherical coupling optical fiber probe consists of incident fiber, effluent fiber and a spherical coupler combining double fibers fixed on the probe tip. The laser beam passes through the coupling lens to enter the coupler and comes out from the effluent fiber in the reverse direction. The return light passes through an object lens and is captured by a CCD camera with an objective lens. This method extends the range of inspecting depth of micro parts, but how to realize three-dimensional measurement capacity and fabricate a smaller fiber coupler with a high coupling efficiency is still an open issue.
To further extend the inspecting depth of micro parts, Jiubin Tan, Fei Wang and Jiwen Cui have invented a fiber probe based on micro focal-length collimation. A cylindrical lens with a focal length in micrometers is formed by a thin glass fiber stylus without coating. A parallel light source is focused by an objective lens to form a point light source. Then, the point light source is collimated by the cylindrical lens and the image fringe is acquired by a linear or area array CCD camera. The probing system has a displacement magnification of larger than 10,000 because the focal length of fiber cylindrical lens is very short. Light propagates outside parts and the measurable depth can thus be extended. However, the limitations of this approach include that the z-displacement is detected by buckling, which is not stable and may be hard to achieve true three-dimensional measurements.
For fiber probes, it is a challenge to make them sensitive to the z-displacement until FBG probe was invented by H Ji, H-Y Hsu, L X Kong and A B Wedding. Their probe comprises FBG in the stylus, and the contact displacements are transformed into the shifts of the center wavelength of reflection spectrum. This probe cannot be affected by shadow effect; theoretically, light can disregard the aspect ratio of micro parts and propagate in the probe. The size of probe and probing system is also miniaturized. When the probe gets contact with parts in the z-direction, FBG is subjected to compression stress and the z-displacement can be readily measured. However, this probe is not sensitive to radial contact displacements because FBG is located in the neutral stress plane when it is deflected.
Above all, fiber probes have been widely applied for measurement of micro parts and become more suitable for its optical and mechanical features of optical conductivity, easy miniaturization and low probing force. Different methods have been designed for sensing the contact displacement of the fiber probe, and the followings are some of their drawbacks:
1. The inspecting depth is restricted by shadow effect. For some probes based on light backscatter, the emission light is easily obstructed or reflected by the sidewall, and a large range of the emission angle allows few particles of light to reach the photo-detector.2. Bulk size of probing system can hardly meet the requirement of the probing space and limits its application for the measurement of micro structures on a complex-shaped part with limited probing space.3. Absence of multi-dimensional tactile sense and multi-dimension-decoupling capacity makes the measurement process complex and time-consuming. A real-time application can hardly be achieved.4. The inspecting resolutions of the fiber probes are hard to be enhanced further. Most of the fiber probes have sub-micrometer resolutions only. The displacement sensitivities are too low to achieve a precise measurement.